Do sub-critical clusters and critical nuclei need to have the same ‘form’?

The answer, of course, is no. But, where’s the evidence?

First, some background. Since nucleation involves a free energy barrier, one invokes certain mythical fluctuations that (somehow!) build up to yield viable nuclei. Usually, the precise details of these fluctuations are brushed aside; in particular, one may be left with the impression that a sub-critical cluster is the same as the viable nucleus, only smaller. Really, though, there’s nothing that precludes the possibility that clusters at different sizes may actually have different ‘forms’ (to be more precise, different crystal structures). But do we have any evidence for this possibility?

We do, thanks to a recent Nature Physics paper by Korean researchers S-Y. Chung, Y-M. Kim, J-G. Kim and Y-J. Kim, who present some really nifty evidence to show that a cluster could — and does! — evolve through multiple forms as it gorws to become a viable nucleus!

The researchers used in-situ high resolution electron microscopy to study early stages of crystallization of amorphous LiFePO4; their method allowed them to not only catch nuclei being formed over several minutes, but also capture structural changes in a cluster while it was on its way to nucleus-hood!

The paper itself is here, and there’s also a commentary by Simon J. L. Billinge. Here’s how they describe what they did:

Taking amorphous LiFePO4 as a multi-component model compound in this study, we revealed the existence of intermediate metastable crystalline states during crystallization. For direct atomic-level observations, we used in situ high-resolution electron microscopy (HREM) at a high temperature. Recent progress in HREM has led to enhanced resolution for rapid and clear imaging even at elevated temperatures. Thus, it enables observation of structural variations in real time in a variety of nanoscale materials […]. HREM image processing […] based on two-dimensional electron crystallography was also used to determine the atomic arrays of intermediates phases, showing the different crystallographic characteristics between each phase.

And here’s the abstract :

Although the classical picture of crystallization depicts a simple and immediate transformation from an amorphous to a crystalline phase, it has been argued that, in selected systems, intermediate metastable phases exist before a stable state is finally reached. However, most experimental observations have been limited to colloids and proteins, for which the crystallization kinetics are fairly slow and the size is comparatively large. Here, we demonstrate for the first time in an inorganic compound at an atomic scale that an amorphous phase transforms into a stable crystalline state via intermediate crystalline phases, thus directly proving Ostwald’s rule of stages. Through in situ high-resolution electron microscopy in real time at a high temperature, we show the presence of metastable transient phases at an atomic scale during the crystallization of an olivine-type metal phosphate. These results suggest a new description for the kinetic pathway of crystallization in complex inorganic systems.

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About Abi

My name is T. A. Abinandanan, and I am a professor of Materials Engineering at the Indian Institute of Science, Bangalore.
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